1. Field of the Invention
The invention relates to apparatus and method for detecting the level of a conductive fluid in a container and more specifically to such a system which has sufficient sensitivity to differentiate between liquid and foam or scale deposits between the probe and the container.
2. Description of the Prior Art
Liquid levels can be controlled by sensing the electrical conductance of the liquid. In a simple conductance level control the equivalent liquid resistance can be used to generate a signal to indicate the presence or absence of liquid at the probe with the probe placed at a critical level in a container.
In McDonnel and Miller's PS800 Series Probe Controls for example, an effective liquid resistance of up to 3,000 ohms can be distinguished from an open circuit at the probe, i.e. no liquid present. However, such simple probe controls cannot readily distinguish resistance of the liquid from resistance due to current leakage from fouling deposits on the probe surfaces or due to additives which may foam in the water.
In most conductance probe controls the probe is connected in series with a resistance network to monitor voltage across the probe as a function of the liquid impedance at the probe. Most such controls depend on a fixed comparison voltage level which determines the decision point value for the control of the output, established for example by a resistance network at the input, to a transistor or amplifier stage. The amplifier stage effectively starts to conduct at an input voltage determined by the characteristics of the stage itself. This fixed voltage level corresponds to a fixed limiting probe impedance value. Thus when the leakage resistance becomes less than about 3,000 ohms the circuit continues to indicate the presence of liquid at the probe, whether this is caused by liquid or only by fouling or foaming leakage currents. Cleaning and maintenance of the probe is then required to correct this condition.
In a boiler the water impedance seen by the probe can vary over wide limits depending on the concentration of contaminants such as boiler compounds, rust, calcium compounds, or other pollutants. City water for example, may measure 500 to 1,100 ohms using a typical conductance probe. However, in some instances, impedance of less than 10 ohms at the probe has been noted. Especially for such low water impedance, the ability of a control with a fouled probe to distinguish between water present or not present at the probe becomes very difficult due to the conduction of current through wet fouling on the probe surfaces.
It has been observed that the impedance of a heavily fouled probe in ohms increases by 5 to 10 times when the probe is out of water. For example, with a water impedance of 100 ohms the probe impedance with a low water level could be 500 to 1,000 ohms. For comparison, with the cited PS800 type control, such a probe would seem to remain in water due to its 3,000 ohm sensing limit being higher than the actual probe impedance.
Prior art patents include U.S. Pat. No. 3,119,266 Atkinson, U.S. Pat. No. 4,027,172, Hamelink and U.S. Pat. No. 4,390,793 John, all of which disclose probe controls which use a three electrode probe and none of which disclose storing and comparing a probe signal with a previously stored probe signal.
U.S. Pat. No. 4,019,067 Gladstone and U.S. Pat. No. 4,263,587 John disclose probe controls in which the probe signal is compared with a fixed reference which is a fixed characteristic of the amplifier stage used. None of the foregoing disclose a sensing circuit which has sufficient sensitivity to distinguish between the presence or absence of a liquid between the probe and the container and of foam or scale deposits therebetween.